1. Field of the Invention
The present invention is related to an output stage circuit, and particularly to an output stage circuit for outputting a driving current that varies with a process.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating an output stage circuit 100 according to the prior art. The output stage circuit 100 includes an N-type metal-oxide-semiconductor transistor 102 and a P-type metal-oxide-semiconductor transistor 104. The N-type metal-oxide-semiconductor transistor 102 has a drain terminal coupled to a last stage circuit 103, a gate terminal for receiving a kick signal kick, and a source terminal coupled to ground GND. The P-type metal-oxide-semiconductor transistor 104 has a source terminal for receiving a first voltage VDD, a gate terminal coupled to the drain terminal of the N-type metal-oxide-semiconductor transistor 102, and a drain terminal coupled to a feedback circuit and/or a load 106 for outputting an output voltage Vx.
When the kick signal kick is enabled, the N-type metal-oxide-semiconductor transistor 102 is turned on, so that a voltage of the drain terminal of the N-type metal-oxide-semiconductor transistor 102 is pulled down to the ground GND. Meanwhile, because a voltage of the gate terminal of the P-type metal-oxide-semiconductor transistor 104 (the voltage of the drain terminal of the N-type metal-oxide-semiconductor transistor 102) is pulled down to the ground GND, the P-type metal-oxide-semiconductor transistor 104 is turned on, resulting in the output voltage Vx being pulled up and a driving current I flowing through the feedback circuit and/or the load 106.
Because the voltage of the gate terminal of the P-type metal-oxide-semiconductor transistor 104 is pulled down to the ground GND, the voltage of the gate terminal of the P-type metal-oxide-semiconductor transistor 104 can not vary with a process of the P-type metal-oxide-semiconductor transistor 104. The driving current I is determined by equation (1):
                    I        =                              1            2                    ×          kp          ×                      W            L                    ⁢                                    (                                                V                  SG                                -                Vt                            )                        2                                              (        1        )            
As shown in the equation (1), kp is a constant value, W is width of the P-type metal-oxide-semiconductor transistor 104, L is channel length of the P-type metal-oxide-semiconductor transistor 104, VSG is a voltage drop between the source terminal and gate terminal of the P-type metal-oxide-semiconductor transistor 104, and Vt is a threshold voltage of the P-type metal-oxide-semiconductor transistor 104. As shown in the equation (1), kp, W, L, and VSG are known, so the driving current I is a constant value. Thus, the driving current I not varying with the process of the P-type metal-oxide-semiconductor transistor 104 may damage the feedback circuit and/or the load 106.